1. Field of the Invention
The present invention relates to an injection molding system. In particular, the present invention relates to an injection molding system with a frequency converter.
2. Description of the Related Art
Injection molding machines are widely used in modem industries and consume considerable electricity. The expenditure in electricity is about 30% of the overall cost of production. Different procedures require different amounts of energy, wherein the energy consumed by some procedures may be ten or more times of that by other procedures. It is therefore the most important issue to the manufacturers to reduce the energy consumption. FIG. 4 of the drawings illustrates a conventional injection molding system using a frequency converter. The injection molding system comprises an injection molding machine 1′, an injection molding machine controller 11′, a pressure valve 3′, a frequency converter 2′, a frequency converter controller 21′, a motor 4′, and a hydraulic pump 5′. The frequency converter controller 21′ is connected to the injection molding machine controller 11′. The injection molding machine controller 11′ is operable to send commanding signals to activate the frequency converter controller 21′ for controlling the frequency converter 2′, which, in turn, outputs low-frequency signals to control the motor 4′ and the hydraulic pump 5′. The speeds of the motor 4′ and the hydraulic pump 5′ can be reduced to lower the energy consumption.
However, since the conditions for injection molding are set by the operator, the speed of the motor 4′ is low when at a small flow operation. The speed of the motor 4′ becomes low, which would cause overheating of the motor 4′ after a long-term operation, leading to damage to the motor 4′. Thus, the low-speed operation of the motor 4′ by the client is so arbitrary that damage to the motor 4′ occurs easily. In a case that the operator increases the frequency, the energy-saving effect is adversely affected.
Normal operation of the hydraulic pump 5′ has a minimum speed limitation and a maximum speed limitation. In a case that the low-speed operation is set by the operator (or the client), the life of the hydraulic pump 5′ is shortened and the hydraulic pump 5′ is apt to malfunction if the operating parameters are inappropriate.
The injection molding machine 1′ and the frequency converter 2′ are separately controlled by the injection molding machine controller 11′ and the frequency converter controller 21′. Referring to FIG. 5, since the injection molding machine controller 11′ has to firstly send commanding signals to control the frequency converter controller 21′, and then the frequency converter controller 21′ controls the frequency converter 2′, there is a lag in time. As a result, the slope of motion of the injection molding machine 1′ is different from that of the frequency converter 2′ when the injection molding machine 1′ speeds up. The lag and the different slopes of motion cause product deficiency. The responding speed is low and the production cycle is long.
Further, the minimum speed limitation to the frequency converter 2′ could not be overcome. As illustrated in FIGS. 4 and 6, when in a low frequency state of the frequency converter 2′, the pressure pulse is large and the pressure changes when the flow changes. Stabilization of pressure by maintaining the pressure directly affects surface mass and internal pressure of the products. However, the pressure is unstable and the product quality is adversely affected if the output frequency of the frequency converter 2′ is set too low.
Whenever the pressure of the injection molding machine 1′ changes during any operation, it is impossible to make the frequency converter 2′ to output frequency in response to different machine types, different pressures, and/or different flow needs. In other words, the speed of the motor 4′ cannot be increased to reduce the formation period during low-pressure operation.
In a high-pressure low-flow operation in high-precision injection molding process, since the frequency converter 21′ receives the commanding signal regarding speed from the injection molding machine controller 11′, the motor 4′ and the hydraulic pump 5′ operate at a low speed, which is detrimental to the motor 4′ and the hydraulic pump 5′. The pressure of the hydraulic system is also unstable, failing to provide complete injection molding. As a result, use of the frequency converter 2′ could not be used in high-precision injection molding.